1. Field of the Invention
The present invention relates to apparatus and methods for inspection or review of semiconductor wafers and masks.
2. Description of the Background Art
The generation of electron optical imaging systems which utilize electrons reflecting of the surface of a flat substrate are complicated when compared to conventional straight-axis electron beam systems. This situation is present because the electron beam passes twice through one or more electron lenses. A design including a plurality of lenses arranged along one straight axis is therefore not practically feasible, and a beam separator is needed to split the incoming and outgoing beams.
A conventional method of low-energy electron microscopy utilizes a prism with a single shaped magnetic field as a beam separator. For example, see E. Bauer, “Low energy electron microscopy,” Rep. Prog. Phys. 57 (1994), p. 895. However, such a beam separator disadvantageously has uncorrected astigmatism.
This deficiency may be compensated for by using a corrected prism design. See V. Kolarik et al., “Close packed prism arrays for electron microscopy,” Optik 87 No. 1 (1991), p. 1; H. Rose et al., “Outline of a versatile corrected LEEM,” Optik 92 No. 1 (1992), p. 31; and U.S. Pat. No. 5,319,207 to Rose et al. (the Rose et al. patent).
For example, FIGS. 1, 2a, and 2b of the Rose et al. patent depict a design that utilizes two uniform fields to reduce astigmatism in the imaging. However, that design is problematic and disadvantageous in that a very high level of accuracy of the inner and outer sector lengths is required in order to achieve stigmatic imaging.
As another example, FIG. 6 of the Rose et al. patent depicts a design that uses opposed uniform fields. However, that design has a complex geometry that is difficult to fabricate and is dispersion free, which in some cases is undesirable.